1. Field of the Invention
The present invention generally relates to a transmission technique for multiplexing signals by means of what is called a spread spectrum modulation, and more particularly to a multiplex communication method of multiplexing signals, which are spread-spectrum-modulated by a plurality of transmitting apparatuses, and spread spectrum transmitting and receiving apparatuses. 2. Description of the Related Art
A spread spectrum modulation is a modulation method of transmitting a signal to be transmitted in a band width wider than a band width which is required to transmit the signal to be transmitted. More concretely, this technique is a technique to further modulate an ordinary modulated signal, which has been primarily modulated, by use of a special pseudo-random (PN) signal (i.e. a spectrum cyclic code), so as to spread the spectrum distribution. This technique is explained, for example, in Japanese Patent Application Laid Open No. Hei.5-22251, in detail.
The basic operation of the spread spectrum modulation and demodulation, is explained here with referring to FIG. 1. A signal to be transmitted, which is inputted through a microphone for example, is modulated to be a primarily modulated signal k. This modulation may be performed by means of various analog to digital modulation method the AM (Amplitude Modulation) method, the FM (Frequency Modulation) method, the ASK (Amplitude Shift Keying) modulation method, the FSK (Frequency Shift Keying) modulation method. This primarily modulated signal k has a band width as shown in FIG. 1. As shown in FIG. 1, the signal k has a relatively sharp signal peak at a frequency fc of the carrier wave, and has a relatively narrow band width. Next, the spread spectrum modulation is performed by use of a cyclic code, which is a pseudo-random signal, and a secondarily modulated signal 1 having a band width as shown in FIG. 1, is transmitted from a transmitting apparatus. As shown in FIG. 1, the secondarily modulated signal 1 has a signal peak smaller than the primarily modulated signal k at the frequency fc of the carrier wave, and has a band width wider than the primarily modulated signal k. Then, a receiving apparatus receives this secondarily modulated signal l and spread-spectrum-demodulates this received signal by use of the same cyclic code as the one used on the side of the transmitting apparatus, so as to obtain a primarily demodulated signal m as shown in FIG. 1, which is substantially the same as the primarily modulated signal k. Then, this primarily demodulated signal m is further demodulated to be an audio signal and outputted as audio sound from a speaker.
In an ordinary transmission path, the transmitted signal is influenced by a disturbance such as a noise. Thus, in case of the transmitted signal having a relatively narrow band width such as the signal k or the signal m shown in FIG. 1, the original signal cannot be recovered if noise exists at the vicinity of the carrier wave frequency fc. However, in case of the transmitted signal having the relatively wide band width such as the signal 1 shown in FIG. 1, the whole portion of the transmitted signal is not lost and the transmitted signal can be recovered even if the transmitted signal has a signal level lower than the noise level. In this manner, the spread spectrum modulation has a good characteristics of opposing against the noise or external disturbance, a good characteristic of having a small signal spectrum density in the transmission path and a good characteristic of keeping the secret of the signal transmission since the spectrum is spread by virtue of the secondary modulation.
There is a first type of multiplex communication system by means of the above explained spread spectrum modulating and demodulating method, in which the signals emitted from a plurality of transmitting apparatuses are multiplexed and transmitted through a transmission path. In each of the transmitting apparatuses, the data to be transmitted is primarily modulated and is then secondarily modulated on the basis of the cyclic code to be outputted onto the transmission path, so that the secondarily modulated signals outputted from the transmitting apparatuses are multiplexed on the transmission path. The cyclic codes used in the transmitting apparatuses are different from each other.
The receiving apparatus receives these multiplexed signals. The received signals are demodulated to be the primarily modulated signals on the basis of the cyclic code generated in the receiving apparatus. Then, it is further demodulated to be the original signals. The transmitting apparatuses prepare pseudo-random codes (e.g. gold code, M-sequence code) different from each other as cyclic codes. In the receiving apparatus, a plurality of cyclic codes are prepared, which correspond to the cyclic codes used in the transmitting apparatuses, and are generated in synchronization with the phase, such that only one of the cyclic codes is sequentially selected.
However, this first type of multiplex communication system has such a problem that a cross correlation between the different channels is considerably large. FIG. 2 shows such a cross correlation between the M-sequence codes having the code patterns different from each other in case where the M-sequence codes are utilized as the spectrum cyclic codes. The abscissa represents the chip number (bit number), wherein the chip represents the phase of the cyclic code for the spread spectrum and indicates the phase difference of the 1 bit clock when it is returned to the initial value after cycling by predetermined bit numbers.
Here, the correlation means such a relationship between two pseudo-random codes that those two codes are coincident to each other at each moment of time. The cross correlation means the correlation between codes which have the code patterns different from each other. The self correlation means the correlation between the codes having the code patterns same to each other. The degree of the correlation is expressed by the correlation value. This correlation value is expressed as a value which is obtained by subtracting the number of the inconsistent bits from the number of the coincident bits when the code sequences are compared with each other as for each bit. As this correlation value becomes larger, the correlation of the compared codes becomes stronger. As the correlation value becomes smaller, the correlation becomes weaker. Further, as the correlation value of the self correlation becomes higher, the demodulation of the transmitted signal becomes easier. As the correlation value of the cross correlation becomes lower, the mutual interference becomes smaller. Therefore, it is the better as the self correlation of the spread spectrum modulated signal and the cyclic code of the transmission object is the larger while the cross correlation is the smaller in relationship with the other spread spectrum modulated signal.
In FIG. 2, the amplitude on the coordinate axis represents the strength of the correlation, and the abscissa represent the shift in the phase of the coded signals between the M-sequence codes having code patterns different from each other. According to FIG. 2, the considerable magnitude of cross correlation exists regardless of the value of the phase (i.e. the chip or bit), and the strong correlation is appeared by a certain interval.
It is therefore concluded here that the correlation between the codes having code patterns different from each other is too strong according to this type of system.
Further, the gold code is the typical code for the spread spectrum modulation. However, the cross correlation between the spread-spectrum-modulated signals having the code patterns different from each other cannot be concluded enough in the gold code although the gold code is slightly superior to the M-sequence code. If the gold code is used in order to improve the cross correlation value, the randomization characteristic is degraded in turn. Here, the randomization characteristic means how much the codes are randomized. Furthermore, in the ordinary spread spectrum transmission, a more special code may be utilized. However, the randomization characteristic of such a special code is even worse than this gold code.
There is a second type of multiplex communication system by means of the aforementioned spread spectrum modulating and demodulating method, which has a basic construction same as that of the above explained first type. However, in this second type, the cyclic codes used in the transmitting apparatuses are synchronized to each other by virtue of a synchronization clock commonly supplied to all of the transmitting apparatuses. Thus, the mutual disturbances between the transmitted signals, which have been cyclic-coded, are restrained because the synchronization is achieved between the transmitting apparatuses.
In the above multiplex communication system, the transmission path may be a wire type such as a cable, or a wireless type such as an electric wave transmission in practice.
However, in this second type of multiplex communication system, if the synchronization cannot be precisely achieved between the transmitting apparatuses, the cross correlation value is changed in the same manner as the case of the aforementioned first type of multiplex system.
Furthermore, in the first and second types of multiplex communication systems, when the jitter or fluctuation is generated in the clock for code generation in the transmitting apparatuses, the cross correlation value is even more changed so that the transmission characteristic becomes unstable, which is another serious problem.
In addition, these first and second types of multiplex communication systems by means of the spread spectrum modulation are explained in more detail in Japanese Patent Application Laid Opens No. Sho. 62-45233 and No. Sho. 6223634, in which the gold code is utilized as a typical cyclic code for the spread spectrum modulation.